A woman reading a Kindle ebook on a London tube train. Amazon.co.uk says downloads have overtaken print book sales. Photograph: Sarah Lee for the Guardian

Amazon.co.uk has said that sales of its Kindle ebooks are now outstripping its sales of printed books.

Underlining the speed of change in the publishing industry, Amazon said that two years after introducing the Kindle, customers are now buying more ebooks than all hardcovers and paperbacks combined. According to unaudited figures released by the company on Monday, since the start of 2012, for every 100 hardback and paperback book sold on its site, customers downloaded 114 ebooks. Amazon said the figures included sales of printed books which did not have Kindle editions, but excluded free ebooks.

Much to the consternation of the publishing industry, Amazon has refused to release audited figures for its digital book sales, something it does for printed books. It told the Guardian that the company would not discuss future policy on the matter.

The company said its figures also showed that British Kindle users were buying four times as many books as they were prior to owning a Kindle, a trend it described as a renaissance of reading.

"As soon as we started selling Kindles it became our bestselling product on Amazon.co.uk so there was a very quick adoption … [And they] are buying four times more books prior to owning a Kindle," an Amazon spokeswoman said. "Generally there seems to be … a love of a reading and a renaissance as a result of Kindle being launched."

Despite revealing that more than half a million Kindle books are priced at £3.99 or less, Amazon said a boost in ebook sales was not just about cheap books and argued that much of its printed range was also sold at a low price.

Ebook sales have been given a boost by the publication of Fifty Shades of Grey by EL James, which has sold two million copies in the past four months.

Three of the top 10 most popular Kindle authors of 2012 – Nick Spalding, Katia Lief and Kerry Wilkinson – were published by Amazon's own Kindle Direct Publishing.

Jorrit Van der Meulen, vice-president of Kindle EU, said: "Customers in the UK are now choosing Kindle books more often than print books, even as our print business continues to grow. We hit this milestone in the US less than four years after introducing Kindle, so to reach this landmark after just two years in the UK is remarkable and shows how quickly UK readers are embracing Kindle. As a result of the success of Kindle, we're selling more books than ever before on behalf of authors and publishers."

• This article has been amended to stress that figures quoted solely reflect Amazon sales